I remember a surprisingly interesting website on nails (and I think screws too) and the defects in them from manufacturing problems. The guy was an expert on the manufacturing process. Your giving me ideas now. The ends of bricks on our house (which is as basic as a 1940s house gets, no fancy brickwork here unfortunately) have finger prints in them. I stumbled across some at a demolition yard with the owner cleaning up a load that were apparently made by prisoners a long time ago - a special triangular symbol was stamped into them. That's when the obsession started. Just picked up a few nice ones with a giant W stamped into them which would never normally be seen as its on the top where the mortar goes. The building was demolished in the Christchurch earthquake, so its got that bit of history as well.